Whatever their other differences, Washington and Berlin are fully in agreement that Turkey has to be prevented from staging an incursion into the Kurdish areas of northern Iraq.

The dispute is one in which the Germans can bring surprisingly effective diplomatic firepower to bear. Indeed, they have already done so.

Germans and Americans are coming at the issue from entirely different standpoints. President Bush is concerned that the arrival of Turkish troops could threaten America's fragile alliance with the Kurdish fighters of northern Iraq, and undermine the US war effort.

By contrast, the German chancellor, Gerhard Schröder, is worried that it could upset another, equally delicate, balancing act - the one which has enabled him to fulfil his campaign pledge to keep German troops out of what he denounced as a "military adventure" while, at the same time, providing the US-led coalition with valuable assistance.

As anyone arriving at Ramstein, the huge US air base amid the wooded hills of the southern Rhineland, can see, German air space and American military facilities on German soil are being used extensively to ferry men and materiel to and from the conflict zone. Not only that, but the German armed forces have taken over responsibility for guarding bases like the one at Ramstein, thus freeing up US troops for a more direct participation in the invasion of Iraq.

The German government has dispatched a specialist anti-nuclear, chemical and biological warfare unit to Kuwait. And it has lent Turkey Patriot missile systems and kept German crew members on the Nato AWACs surveillance planes patrolling Turkish airspace.

None of this, according to Mr Schröder, constitutes participation in the war. Letting the Americans use their bases and German air space is merely a question of a commitment to an ally (even though most experts agree that the German government could legally block access to both).

The unit in Kuwait is explained away, rather less than credibly, as being there in support of the "war on terror", which Berlin supports, rather than the assault on Iraq, which it does not. As for the AWACs crews and Patriot missiles, they have been sent to Turkey, a fellow-Nato member, under the article in the alliance's treaty that obliges each of its members to help defend another if required to do so.

This last is the nub of Mr Schröder's problem. If, now, Turkey sends troops over the border, Germany will no longer be defending an innocent bystander, so to speak, but a country that has made itself a party to the conflict.

That would be unacceptable, not only to most of the German public, which is overwhelming against this war, but more pertinently to many of the Social Democrat and Green MPs who keep Mr Schröder in power and who are already fretting over the overflight rights accorded to the US.

Last weekend, the government in Berlin said that if the Turks carried out their plans, it would demand the return of the Patriots and pull its crew members off the AWACs. Germans are reckoned to make up about a quarter of the personnel and according to reports this week in the German media their withdrawal could force the entire mission to be abandoned.

What effect that would have on the overall conduct of the war is unclear. It has been speculated that the AWACs are doing rather more than just protecting Turkish territory and have a key role to play - whether actual or potential - in coordinating operations in Iraq itself.

For the moment, a crisis has been averted. At a meeting of Nato ambassadors on Monday, the German representative accepted assurances from his Turkish counterpart that any additional Turkish troops - there have been 'military advisers' and gendarmerie units in northern Iraq for several years - would play a non-offensive role.

Ankara's line is that they would be there to pre-empt a flood of refugees into Turkey and stop guerrillas from the radical Kurdish PKK movement from mounting insurgency operations in the same direction.

But whether the second of those missions can really be achieved without bringing Turkish troops into conflict with other Kurdish elements remains to seen. The situation is still immensely sensitive. And it could become more so at any moment.